Hitherto, image forming apparatuses, such as copying apparatuses, printers, facsimiles, and multi-functional peripherals of these apparatuses, have been known as being capable of forming (printing) an image on transfer materials, such as a sheet of paper. To fix an image to transfer material, the image forming apparatuses include fixing devices that have a heating rotating member, a pressing rotating member, and a heater, such as a halogen heater. The pressing rotating member and the heating rotating member form a fixing nip section for nipping the sheet of paper to which a toner image is transferred. The heater heats the heating rotating member.
In recent years, as a method of heating the heating rotating member in the fixing device, a method of heating the heating rotating member by induction heating (IH) that is achieved by electromagnetic induction may be used in addition to a method of heating the heating rotating member by using a halogen lamp. In the induction heating (IH), the fixing device includes a heating rotating member that generates heat by the induction heating, an induction coil that causes generation of magnetic flux for heating the heating rotating member by the induction heating, and a magnetic member core (magnetic member core section) that forms a magnetic path serving as a path for the magnetic flux generated by the induction coil. Comparing with the heating method of using a halogen lamp, the induction heating is advantageous in that it is capable of quick heating and provides high heating efficiency.
For the fixing device that uses the induction heating, various technologies are developed for suppressing an excessive temperature increase of the heating rotating member at an area (non-feeding area) that is situated outwardly of a feeding area where a sheet of paper passes. The suppression of the excessive temperature increase is conducted in accordance with a length (sheet feed width) in a paper-width direction of the sheet of paper that is transported (fed) to the fixing device. The sheet-width direction corresponds to a direction vertical to a direction of transportation of the sheet of paper. In particular, a fixing device is proposed to have the capability of adjusting the heating value of the heating rotating member at the non-feeding area and at the feeding area in the paper width direction in accordance with paper size.
The proposed fixing device includes a heating rotating member that generates heat by induction heating, a pressing rotating member that forms a fixing nip section with the heating rotating member, an induction coil that causes generation of magnetic flux, a magnetic member core (magnetic member core section) that forms a magnetic path serving as a path for magnetic flux generated by the induction coil, and a magnetic flux shielding member (magnetic member core section) that reduces or blocks the magnetic flux.
The magnetic member core of the proposed fixing device includes a center core (second core section), an arch core (first core section), and a side core (third core section). The center core forms a magnetic path near an inner peripheral edge of the induction coil, and has a surface opposing an outer peripheral surface of the heating rotating member without the induction coil being interposed therebetween. The arch core opposes the outer surface of the heating rotating member with the induction coil being interposed therebetween. The side core forms a magnetic path near an outer peripheral edge of the induction coil.
In the proposed fixing device, at the area that is situated outwardly of the feeding area where the sheet of paper passes, the magnetic flux shielding member is movable between a shielding position disposed between the center core and the heating rotating member and a non-shielding position that is not disposed between the center core and the heating rotating member. When the magnetic flux shielding member is positioned at the shielding position disposed between the center core and the heating rotating member, the magnetic flux shielding member opposes the outer peripheral surface of the heating rotating member, so that the magnetic flux is reduced or blocked. In such a fixing device, the center core and the arch core are disposed apart from each other by a distance that allows passage of the magnetic flux shielding member. The magnetic flux shielding member is movable between the center core and the heating rotating member.
Therefore, in the proposed fixing device, compared to when the center core and the arch core are integrated to each other or when the center core and the arch core are disposed in contact with or close to each other, the degree of coupling of a magnetic field between the center core and the arch core tends to weaken. Therefore, when the center core and the arch core are disposed apart from each other, heating efficiency of the heating rotating member is reduced compared to when the center core and the arch core are integrated to each other or when the center core and the arch core are disposed in contact with or close to each other.
Consequently, hitherto, in order to increase the heating value of the heating rotating member, the diameter of the induction coil is formed to a predetermined diameter. However, when the diameter of the induction coil remains the same as before, the heating rotating member needs to be large. As a consequence, the heat capacity of the heating rotating member is increased, and warm-up time may be increased, that is, the time from when the heating of the fixing device is started to when the fixing device becomes usable may be increased. Thus, there is a demand for a fixing device that can suppress a reduction in the heating efficiency of the heating rotating member.